BRITISH ASSOCIATION. 31 



this country to maintain her present supremacy in manufacturing industry. So 

 far as this particular district is concerned, it is generally admitted that 200 years 

 will be sufficient to exhaust the principal seams even at the present rate of 

 working. If the production should continue to increase, as it is now doing, the 

 duration of those seams will not reach half that period. How the case may stand 

 in other coal-mining districts I have not the means of ascertaining ; but as the 

 best and most accessible coal will always be worked in preference to any other, 

 I fear the same rapid exhaustion of our most valuable seams is everywhere 

 taking place. "Were we reaping the full advantage of all the coal we burnt, no 

 objection could be made to the largeness of the quantity, but we are using it 

 wastefully and extravagantly in all its applications. It is probable that fully 

 one-fourth of the entire quantity of coal raised from our mines is used in the 

 production of heat for motive power ; but, much as we are in the habit of admir- 

 ing the powers of the steam-engine, our present knowledge of the mechanical 

 energy of heat shows that we realize in that engine only a small part of the 

 thermic effect of the fuel. That a pound of coal should, in our best engines, pro- 

 duce an effect equal to raising a weight of a million pounds a foot high, is a result 

 which bears the character of the marvellous, and seems to defy all further 

 improvement. Yet the investigations of recent years have demonstrated the fact 

 that the mechanical energy resident in a pound of coal, and liberated by its com- 

 bustion, is capable of raising to the same height 10 times that weight. But 

 although the power of our most economical steam-engines has reached, or perhaps 

 somewhat exceeded', the limit of a million pounds raised a foot high per lb. of 

 coal, yet, if we take the average effect obtained from steam-engines of the various 

 constructions now in use, we shall not be justified in assuming it at more than 

 one-third of that amount. It follows, therefore, that the averag* quantity of coal 

 which we expend in realizing a given effect by means of the steam-engine is about 

 30 times greater than would be requisite with an absolutely perfect heat-engine. 



The causes which render the application of heat so uneconomic in the steam- 

 engine have been brought to light by the discovery of the dynamical theory of 

 heat ; and it now remains for mechanicians, guided by the light they have thus 

 received, to devise improved practical methods of converting the heat of combus- 

 tion into available power. 



Engines in which the motive power is excited by the communication of heat to 

 fluids already existing in the aeriform condition, as in thole of Stirling, Ericsson 

 and Siemens, promise to afford results greatly superior to those obtained from the 

 steam-engine. They are all based upon the principle of employing fuel to gen- 

 erate sensible heat, to the exclusion of latent heat, which is only another name 

 for heat which has taken the form of unprofitable motion amongst the particles of 

 the fluid to which it is applied. They also embrace what is called the regenera- 

 tive principle — a term which has, with reason, been objected to, as implying a 

 restoration of expended heat. The so-called " regenerator " is a contrivance for 

 arresting unutilized heat rejected by the engine, and causing it to operate in aid 

 and consequent reduction of fuel. 



It is a common observation that before coal is ezhaasted some other motive 

 agent will be discovered to take its place, and electricity is generally cited as the 

 coming power. Electricity, like beat, may be converted into motion, and both 



